Phonofilm is an optical sound-on-film system developed by inventors Lee de Forest and Theodore Case in the early 1920s.
In 1919 and 1920, de Forest, inventor of the audion tube, filed his first patents on a sound-on-film process, DeForest Phonofilm, which recorded sound directly onto film as parallel lines. These parallel lines photographically recorded electrical waveforms from a microphone, which were translated back into sound waves when the movie was projected.
The Phonofilm system, which recorded synchronized sound directly onto film, was used to record vaudeville acts, musical numbers, political speeches, and opera singers. The quality of Phonofilm was poor at first and while it improved somewhat in later years, it was never able to match the fidelity of sound-on-disc systems such as Vitaphone, or later sound-on-film systems such as RCA Photophone or Fox Movietone.
The films of de Forest were short films made primarily as demonstrations to try to interest major studios in Phonofilm. These films are particularly valuable to entertainment historians, as they include recordings of a wide variety of both well-known and less famous American vaudeville and British music hall acts which would otherwise have been forgotten.